1. Field of the Invention
The present invention relates to a radio communication apparatus utilizing an array antenna and a method of detecting path-timing thereof and particularly to a structure for processing the baseband signal.
2. Description of the Related Art
As the mobile communication system for the next generation, development of a digital cellular radio communication system utilizing the DS-CDMA (Direct Spread Code Division Multiple Access) technology is now under way.
The CDMA system has been proposed to realize simultaneous communications by assigning channels using codes. In this system, the signals from the other channels under simultaneous communications generate interference and thereby the number of channels for simultaneous communications (capacity of channel) is restricted as a result.
That is, in a mobile communication system utilizing the DS-CDMA technology, communication quality (average bit error rate during digital communication) is deteriorated by the amount of interference from the other users having multiple connections and the system capacity is determined based on the number of multiple access users satisfying the predetermined communication quality.
Accordingly, adaptation of interference suppressing technology is effective for increasing the channel capacity.
Adaptive array antenna is a technology which can be adaptively introduced for beam forming to the desired users and null point forming to the user who generates a large interference source and is also a technology to enable an increase in the channel capacity.
Namely, radio signals can be received at higher sensitivity from the desired user by forming the signal beam in the direction of the desired user and directing the null point to the user who is a large interference source.
The channel capacity can be increased by reducing amount of interference as described above.
Moreover, in the CDMA communication system in a mobile communication environment, system capacity may be lowered due to the differences in the distances to the base station from many users who are communicating with only one base station.
Therefore, system capacity can be increased by controlling the transmitting power to equalize the received power of each user of the base station.
When the array antenna is adapted to the DS-CDMA system to execute transmit power control, the transmit power control is performed to make constant the SIR (signal to interference power ratio) after beam forming and RAKE combining.
FIG. 6 illustrates a prior art of a structure of a baseband signal processing unit of the existing DS-CDMA array antenna receiving apparatus shown in Japanese Published Unexamined Patent Application No. 84216/2002.
The radio frequency signal received with each antenna element of the array antenna 201, consisting of a plurality of antenna elements, is converted to the baseband signal in a frequency converting unit and is then converted, by an A/D converter to the discrete/quantized digital baseband signal (not illustrated).
The digital baseband signal is then input to an adaptive array receiving unit 100 for executing the beam forming and demodulation process and a searcher 200 to execute the path-search.
The adaptive array receiving unit 100 is formed of a plurality of fingers 101 for receiving a multi-path signal with the RAKE receiving method and each finger 101 is provided, for the receiving signal process of each path, with a despreading unit 102, a beam former 103 and a coherent detecting unit 104 to execute the receiving signal process of each path.
The RAKE reception is performed by coherently combining the output signals demodulated by the fingers 101 in the RAKE combining unit 108.
Moreover, a weight updating unit 106 using an adaptive algorithm is also included in the adaptive beam forming in accordance with the receiving environment. Since the weight updating algorithm of this weight updating unit 106 is well-known, a detailed description is omitted here.
According to this known weight updating algorithm, the amount of interference can be reduced by directing the beam to the desired direction and moreover directing the null point to the direction of other users having the larger interference power.
The beam former 103 adjusts the relative phase of the received signal or transmitted signal and then changes the direction in which the transmit/receive intensity of the signal transmitted from the array antenna is most intensive (direction of beam forming) by multiplying a weight expressed with a complex number to the signal from each antenna element.
The weight updating unit 106 updates a value of weight used by the beam former 103 and then gives this updated value to the beam former 103 in order to optimize the direction of beam formed by the beam former 103.
Meanwhile, the signals of the array antenna 201 input to the searcher 200 are converted to complex correlation value signals through a correlation process for despreading process for every antenna element with the correlation processing units 212-1, 212-2, 212-3, 212-4 and are then input to an antenna combining path-timing detecting unit 202 and an antenna-to-antenna correlation estimating unit 203.
In the antenna combining path-timing detecting unit 202, the complex correlation value signals present after the correlation process of each antenna are converted to the value of powers by a power value converter 205 and are then combined with a combiner 206.
Accordingly, large signals of almost the same level can be obtained among the antenna elements at the time when the path exists, but when the path does not exist, and noise is appearing, random signals of lower levels are combined. As a result, the signal element is multiplied with the number of antennas, but noise is not multiplied with the number of antennas and is equalized to an average value. Therefore, the distribution of noise for the signal after combining is suppressed in comparison with that before combining.
The signal obtained by combining the complex correlation values for every antenna element is performed to a temporal average by accumulating the values of powers at the predetermined time in an averaging unit 207 and delay profiles are stored into a delay profile unit 208 after the averaging process of variations resulting from fading in each path.
The path-timing detection is executed by searching the path in the larger level in the path detecting unit 209 using the delay profile.
In the antenna-to-antenna correlation estimating unit 203, the antenna-to-antenna correlation value (the cross-correlation value between the received signals by adjacent antenna elements) is estimated in the antenna-to-antenna correlation unit 210 and the estimated antenna-to-antenna correlation value is performed to a temporal average in the averaging unit 211. Thereby, variation in the level of the path due to fading can be compensated.
As the output of the averaging process by the averaging unit 211, an antenna-to-antenna correlation estimating value corresponding to each timing of the delay profile can be obtained.
In the adaptive array receiving unit 100, the phase term information of the antenna-to-antenna correlation estimating value obtained from the searcher 200 is converted to weight information in a weight converting unit 105 and is set as an initial value of the weight updating unit 106.
A plurality of path-timing information pieces detected by the searcher 200 are sent to a finger assigning unit 107 of the adaptive array receiving unit 100 and is used as the despreading timing of each finger 101.
In the prior art, the path-timing detection is performed with a correlation process for each antenna element in the searcher, conversion of the obtained complex correlation values to the powers, and then combining (adding) of these powers.
The reason is that since the complex correlation value of each antenna element generates phase rotation due to variation in frequency (Doppler, carrier frequency variation or the like) of the receiving signal, when the complex correlation values of antenna elements are all added in voltage, namely added as vectors, the values after the combining process may be deteriorated in accordance with the complex correlation values to be added and therefore the complex correlation values of antenna elements cannot be added as vectors.
Accordingly, the complex correlation values must be converted to powers in view of eliminating influence by phase rotation.
As described above, frequency variation must be taken into consideration in the prior art and the averaging process is executed after conversion to power values in the length of a temporal average where frequency variation cannot be neglected.
Moreover, in the prior art, the delay profile is obtained by conducting the temporal averaging process after the correlation process (corresponding to the despreading process) is executed, using a replica of the spread code and the receive signal in the searcher. However, the receive signal vector is rotated by the modulation data.
Therefore, the polarity of symbols must be cancelled in the case where the temporal average process is executed for a plurality of symbols and only the known pilot symbol part can usually be used and only a small number of samples may be used.
Therefore, a need arises for a technique that can realize stable operation of the system and improve system capacity through improvement in the path detection characteristics and that can improve the noise cancellation effect.